1. Field of the Invention
The invention is related to a method for growing group III-nitride crystals in a mixture of supercritical ammonia and nitrogen, and group III-nitride crystals grown by the method.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Gallium nitride (GaN), and its ternary and quaternary alloys containing aluminum and indium (AlGaN, InGaN, AlInGaN), has been used in wide variety of light emitting devices and electronic devices, such as light emitting diodes (LEDs), laser diodes (LDs), microwave power transistors, and solar-blind photo detectors. Some of these devices are already in the market, and widely used in cell phones, indicators, displays, etc.
However, these devices are typically grown epitaxially on heterogeneous substrates, such as sapphire and silicon carbide since GaN wafers are not yet available. The heteroepitaxial growth of group III-nitride causes highly defected or even cracked films, which hinders the realization of high-end optical and electronic devices, such as high-brightness LEDs for general lighting and blue LDs for DVD recording.
All the problems related to heteroepitaxial growth of group III-nitride can be ultimately solved by using group III-nitride single crystalline wafers, which are sliced from bulk group III-nitride crystal ingots. However, it is very difficult to grow a bulk crystal of group III-nitride, such as GaN, AlN, and InN, since group III-nitride has a high melting point and high nitrogen vapor pressure at high temperature.
Up to now, a couple of growth methods using molten Ga, such as high-pressure high-temperature synthesis [1,2] and sodium flux [3,4], have been used to obtain bulk group III-nitride crystals. However, the crystal shape grown in molten Ga becomes a thin platelet because molten Ga has low solubility of nitrogen and a low diffusion coefficient of nitrogen.
On the other hand, growth of group III-nitride crystals in supercritical ammonia has been proposed and researched by several groups [5-10]. This new technique called ammonothermal growth has the potential for growing large bulk group III-nitride crystals, because supercritical ammonia used as a fluid medium has high solubility of source materials, such as group III-nitride polycrystals or group III metal, and has high transport speed of dissolved precursors.
However, state-of-the-art ammonothermal method is limited by the growth rate of the group III-nitride crystal, which impedes the application of this method to industrial mass production. The present invention discloses new findings that are derived by theoretical calculations of the equilibrium molar ratio of NH3, N2, and H2 in the ammonothermal reaction. The new findings from these calculations reveal the weak point of the current ammonothermal method. Based on these new findings, a new approach to solving the growth rate problem of the ammonothermal method is disclosed.